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Project by Tim Blythman USB C SERIAL ADAPTOR USB Type-C (USB-C) was introduced around 10 years ago and is now becoming standard. While USB-serial adaptors with Type-C sockets are available, many do not adhere to the USB-C standard and may also have Windows driver problems. Our design, presented here, has no such drawbacks and is relatively simple and compact. W e have started adding USB-C sockets to our projects as the necessary components have become available in a format that is easy to solder. Because almost all new smartphones and tablets have USB-C sockets, USB-C chargers and cables are becoming commonplace. Small electronic modules have been a great boon for many reasons. In parallel with the rise of Arduino, they have made it very easy to connect microcontrollers to other electronic components. We have a bit of a love/hate relationship with USB-serial adaptors. While they are incredibly useful and inexpensive, sometimes the chips used in them are clones. You might not have any idea of that until a Windows update causes the device to stop working. A clone chip can look identical to the real deal; sometimes, the only way to tell is to X-ray it! It isn’t just a single chip that suffers from this problem. Chips labelled FT232, PL2303 and CH340G have caused problems in the past. Others may be vulnerable too. Our design doesn’t have this problem because we use a PIC microcontroller programmed to act as a USB/ serial bridge, and it identifies as a generic CDC device, so there should be no way that the drivers can go wrong. Windows, Linux and macOS recognise it without needing any special drivers installed and should work immediately after being plugged in. We have used USB-serial adaptor modules based on the CP2102 chip in several projects. We covered this module with a dedicated article in the January 2017 issue (siliconchip. au/Article/10510). One advantage of Fig.1: a USB-C source provides pullup currents, while a sink has pulldown resistors. Both can monitor the voltage on the CC line to determine what has connected to the other end of the cable. The source applies different currents (Ip) depending on its capacity to supply current to VBUS, which the sink can detect as differing voltages on the CC line. Advanced modes, like power delivery (PD) and dual role (DRP), are negotiated through digital signalling on the CC lines. 68 Silicon Chip Australia's electronics magazine the CP2102 is that, like our design, it doesn’t require drivers to work with modern operating systems. Because of that, both the CP2102 module and our version will work if plugged into our Pico Digital Video Terminal from the March and April 2024 issues (siliconchip.au/ Series/413). The most common CP2102 module is a compact device with a micro-USB socket to connect to a computer and a six-pin header to provide 3.3V logic level UART (universal asynchronous receiver transmitter, ie, serial) signals. So we have patterned our designs on that one. USB-C advantages and challenges USB-C is becoming ubiquitous; even Apple products like the iPhone, which have long had proprietary connectors, have switched to using USB-C, starting with the iPhone 15 in 2023. The latest version of the Microchip PICkit debugger and programmer, the PICkit 5, also has a USB-C socket. We think that is an improvement over the micro-USB socket on its predecessor, the PICkit 4. We have reviewed the PICkit 5 in the November 2023 issue (siliconchip.au/Article/16016) Although only slightly larger, in our experience, USB-C plugs and sockets are more robust than the micro-USB and mini-USB parts that preceded them. USB-C plugs and sockets are also symmetrical, which means they are less fussy to use. USB-C to USB-C cables also exist, siliconchip.com.au USB-C Serial Adaptor Features & Specifications ● Drop-in replacement for compact CP2102-based USB-serial modules with the same connector pinout ● Uses the now standard USB-C socket instead of a micro-B USB socket ● Uses a low-cost PIC16F1455 microcontroller with a USB full-speed peripheral ● Moderate component size for hand construction ● Supports 8N1 format and a wide range of baud rates (47 baud to 3Mbaud) ● 3.3V, DTR, RX, TX, GND and 5V connections ● LED indicators for power, data reception and data transmission ● No concerns about Windows drivers refusing to work with it due to counterfeit blocking attempts USB-C Serial Adaptor Kits (SC6652, $20) Includes the PCB, programmed microcontroller and all other parts to build the module; see the parts list later in this article. in which case the cable ends are even interchangeable. They are certainly less bulky than the USB sockets and plugs that appeared over 20 years ago. So it is no surprise that USB-C is becoming popular. USB-C is also more complex than its predecessors and requires some knowledge to implement correctly. That has tripped up some engineers who don’t understand the requirements fully. Even the Raspberry Pi Foundation had trouble with this, as their first release of the Raspberry Pi 4 had a hardware bug that meant it would not work with some USB-C cables, specifically ‘smart’ e-marked (with embedded electronics) cables. Older, simpler legacy cables appeared to be immune. In simple terms, the signalling resistors used to determine the orientation and role of the cable (in combination with the CC wire in the cable) were not connected correctly. This meant that very early versions of the Raspberry Pi 4 boards were identified as audio adaptors instead of devices requesting a 5V power source and thus did not work. Legacy cables, such as USB-A to USB-C types, lack the CC wire in the cable and thus do not respond to the incorrect signalling and deliver power regardless. Fig.1 shows how the signalling should work. There is more background on this at siliconchip.au/ link/abu0 We’ve seen some versions of the CP2102 USB-serial modules that have replaced the micro-USB socket with siliconchip.com.au a USB-C socket but they completely omitted the signalling resistors. That means that these modules will not work in all cases. Such devices may appear to have intermittent faults, working with some cables or hosts but not others. At worst, they might not work at all. Our USB-C Serial Adaptor So, this USB-C Serial Adaptor is a drop-in substitute for the cheap but functional CP2102 USB-serial Module and it actually works reliably! Our Adaptor is a small PCB with a USB-C socket at one end and a sixway header at the other. Unlike the prebuilt modules you can buy, this is a constructional project you must assemble yourself. We have used some small parts, but it should be eminently doable for those with much experience in SMD soldering. It uses a PIC16F1455 microcontroller for its USB interface. The PIC16F145x family is one of the cheapest programmable chips with a USB peripheral. We’ve used the PIC16F1455 in several projects, most The USB-C Serial Adaptor is a minuscule 16×22mm and operates as a dropin replacement for the well-known CP2102 USB-serial Module. Its USB-C socket is more robust and modern than the micro-USB socket on typical USB-serial modules. The components are mostly M2012 (0805) size, but still can be hand-soldered. The USB-C socket is the finest-pitch part, so check its soldering thoroughly before applying power to the board. notably the Microbridge from May 2017 (siliconchip.au/Article/10648). The Microbridge provides a similar USB-serial function as our Adaptor but can also program PIC32 chips. However, the Microbridge doesn’t break out the DTR (data terminal ready) signal like the CP2102 module. The Microbridge also has a different connector pinout, meaning it is not a drop-in replacement for the Module. Circuit details One of the many types of CP2102based modules, which our USB-C Serial Adaptor is meant to replace. Fig.2 shows the circuit diagram of our new Adaptor. The USB socket, CON1, is a USB-C type that lacks the high-speed pairs. That means it only has one row of pins, making it easier to solder. The high-speed pairs are not needed for this design. We previously used a USB-C socket with those extra pins in the USB Cable Tester from the November and December 2021 issues (siliconchip.au/ Series/374). It had two rows of very fine pins and was very fiddly to solder; the variant used in this Adaptor is easier to work with. The SBU (sideband use) pins are present on the connector we’re using, but are not needed in this design and so are not connected. The two CC pins (configuration channel) are each connected to ground via 5.1kW resistors, signalling that the Adaptor is a power sink (ie, it consumes power rather than provides power). The remaining pins on CON1 are duplicated but are otherwise the same as used in standard USB 2.0 applications. The duplicated pins are simply Australia's electronics magazine June 2024  69 Unlike CP2102 modules, the USB-C Serial Adaptor (shown enlarged) has components on both sides, including a 1.27mm (0.05in) pitch 14-pin SOIC chip and a handful of passive components. connected together. They exist because the connector can be plugged in with two different orientations. CON2 is a six-way pin header matching that on the CP2102 modules. It provides a means to connect to the logic-level serial signals. 5V power and ground from CON1 are connected through to CON2, as well as supplying REG1, an MCP1700-3.3V regulator. It, and its two 1μF bypassing capacitors, provide the 3.3V supply to match that on the CP2102 module and so provide 3.3V logic levels. If you just wanted to get 5V and 3.3V from a USB-C cable, you could populate the Adaptor PCB with just the components mentioned so far. PIC16F1455 microcontroller IC1 is powered at pins 1 and 14 from the 3.3V rail. There is no separate bypass capacitor because the circuit is physically very small, and the 1μF capacitor on the 3.3V rail is close to the requisite pins on IC1. As an aside, the PIC16F1454 is much the same as the PIC16F1455, except it lacks the analog peripherals (such as the analog-to-digital converter [ADC]). We are not using any analog features, so the two chips are essentially interchangeable in this role. You should have no trouble using the PIC16F1454 if you have one on hand. Power indicator LED3 is fed from the 3.3V rail via a 1kW current-­limiting resistor. Serial data indicators LED1 (TX) and LED2 (RX) are driven via 1kW resistors from pins 9 and 10 of IC1 (digital outputs RC1 and RC0), respectively. Pin 11 of IC1 is connected to a 100nF capacitor that filters the output of a regulator internal to IC1’s USB peripheral. The USB D+ and D- signal lines (IC1’s pins 13 and 12) connect to the corresponding pins on USB socket CON1 to provide the USB data interface. Pins 5, 6 and 7 on IC1 are connected to CON2 via 220W resistors; these are the UART RX, TX and DTR signals, respectively. The 220W resistors protect the microcontroller by limiting the current that can flow through the pins. The 100kW resistor provides a weak pullup on the RX pin, preventing noise from being seen as data if that CON2 pin is left unconnected. The PIC16F1455 lacks an internal pullup on this pin, so we must provide this externally. Software The USB function is heavily dependent on software. We mentioned the Microbridge earlier; the Adaptor uses the same software library to provide the virtual USB serial port functions. The library enumerates IC1 as a CDC (communications device class) device. CDC encapsulates the features of devices like fax machines and modems that use a serial interface, so it is well suited to working as a virtual USB-serial port. The Adaptor software also configures pins 5 and 6 of IC1 as the UART (universal asynchronous receiver/ transmitter) RX (receive) and TX (transmit) pins. Unlike newer PIC chips, these functions cannot be allocated to other pins. Fig.2: aside from its basic functionality, the USB-C Serial Adaptor provides a few niceties, such as independently-driven TX and RX LEDs, series protection resistors for the data lines and a weak pullup on RX for noise rejection. 70 Silicon Chip Australia's electronics magazine siliconchip.com.au In theory, the Adaptor simply needs to check the current baud rate, take data at that rate from the UART RX pin and send it to the USB host, and from the USB host to the UART TX pin. In practice, a few other things need to happen to make it compatible with the CP2102 module. For a start, LED1 is switched on for about 50ms every time serial data is received from the USB host. Similarly, LED2 switches on whenever data is seen on the UART RX pin. Having separate lines to drive these LEDs means that the TX and RX lines are not loaded unnecessarily. We can also show a clearer indication that data is present by lighting the LED longer than it would be if driven directly by brief pulses on the serial lines. The DTR pin is held at a high idle level and then taken low whenever the virtual USB port is open; this means an application is actively connected to the CDC device. Also, the UART TX pin is set to a high-impedance state if a USB host is not connected. The utility of these functions may not be obvious, but they have specific uses in applications like the Arduino. Arguably, modules like the CP2102 USB-serial adaptor exist because of the Arduino ecosystem. In early Arduino boards (before the Uno!), the DTR pin on a separate USB-serial adaptor was used to reset the microcontroller and enter a bootloader. An RC circuit turns the high-low transition into a brief pulse for the micro’s reset pin, and the bootloader runs for the first second or so after reset. The circuit on the Uno R3 works similarly, although the USB-serial adaptor is incorporated into the board. Allowing the TX pin to float if there is no active connection means the corresponding RX pin on whatever is attached can be used for other purposes when not needed for programming since it is not being driven. USB data is passed in packets at times dictated by the driver in the USB host. Data is sent and received over the bus at 12Mbps (USB fullspeed) during these periods. If transmission and reception are both occurring, this data must be interleaved over the bus. Each direction has a 256-byte buffer to smooth the transition between the packetised USB data and the continuous UART data. The UART peripheral can also buffer a byte or two of siliconchip.com.au data before it gets moved to or from the main buffers. The software also monitors for packets requesting changes in the baud rate or to send a ‘break’ signal. A break is simply a condition where the TX line is held low for a time longer than one byte (the PIC16F1455 does this for 13 bit times). It is often used to synchronise transmission with the receiving device. When a request for a break signal is sent from the computer, the TX LED flashes for half a second. Limitations We have chosen the PIC16F1455 because it is inexpensive, but that is for a reason. An 8-bit microcontroller does not have much processing power, especially for handling the amount of data that USB can move around. As such, the Adaptor cannot do everything that a CP2102 module can. The UART peripheral on IC1 is limited to 8-bit or 9-bit data, and it does not natively support parity bits like the CP2102 chip. To keep things simple, we only support 8-bit mode. This helps with the throughput of the Adaptor too, as there is one less special case to handle. The current version of the software uses 92% of the 1024 bytes of available RAM, so there wouldn’t be space to store the 9th bit for both 256-byte buffers even if we wanted to. Still, it can handle all the typical use cases for a USB-serial adaptor, including very low and very high baud rates. Baud rates The PIC16F1455 has hardware that uses the USB host’s clock to tune its 48MHz internal oscillator; the available steps result in an oscillator error of up to 0.2%. The microcontroller can produce a wide range of baud rates, from 47 to 3,000,000 baud, from the 12MHz instruction clock. Our calculations show that the error in deriving the baud rate will be less than 0.2% for the standard rates shown in Table 1. Thus, the total error in the requested baud rate compared to the actual baud rate will be less than 0.4% for standard rates. Any arbitrary baud rate under 1Mbaud (1,000,000 baud) will have an error of less than 4%, which should be sufficient for most applications over short distances. Australia's electronics magazine Table 1 – baud rate accuracy Baud rate Max. error 110 0.20% 300 0.20% 600 0.20% 1200 0.20% 2400 0.20% 4800 0.20% 9600 0.20% 14,400 0.24% 19,200 0.20% 38,400 0.36% 57,600 0.36% 115,200 0.36% 230,400 0.36% 250,000 0.20% 460,800 0.36% 1,000,000 0.20% Typical error at standard baud rates (including 0.2% due to the internal oscillator). The throughput of a USB full-speed connection is 12 megabits per second; this will not be achieved in practice, as the USB connection is usually shared with other devices. Remember that this also includes data in both directions. In practice, the limit is much lower, primarily due to the drivers that limit the size of the packets that can be sent. We cannot easily change this, so we are somewhat stuck with that. So continuous transmission at higher baud rates is not possible, although we had no trouble sending and receiving bursts of data up to 3Mbaud and continuous reception up to 460,800 baud. Most of these concerns will not affect the common uses of these modules, such as acting as a programming interface for a microcontroller or handling user input (eg, on a Micromite) at baud rates between around 4800 and 115,200. Programming We have omitted a microcontroller programming header to keep the USB-C Serial Adaptor much the same size as the CP2102-based modules. Thus, unless you have a pre-­ programmed microcontroller, you should program it before soldering it to the PCB. If you purchase a kit from the Silicon Chip shop, IC1 will be programmed, so you won’t have to worry about it. June 2024  71 Our PIC Programming Adaptor project from September 2023 (siliconchip. au/Article/15943) has more information about the gear you might need to program an SMD chip. Note that you will also need a PICkit 3, 4 or 5 to do the programming. To allow us to quickly reprogram our prototype during development, we soldered fine wires directly to the PIC’s programming pins while it was mounted on the PCB. That is an option to consider if you only need to do this once for this project. We used the low-voltage programming pins (pins 12 and 13) since the other programming pins (pins 9 and 10) are loaded by the LEDs, which could interfere with programming. Of course, pins 12 and 13 are the USB pins, so you should not have a programmer connected at the same time anything is connected to the USB socket. Fig.3: use this diagram and the photos to ensure that the many small components are all fitted in the correct locations. Take care that IC1 is installed the proper way. If you look from the end of the chip, you should see the chamfered edge on the pin 1 side. a PCB, you might prefer a straight header. If you are adding the Adaptor to a low-power design, you could omit the LEDs to save on the current they would draw. In that case, you could also omit the 1kW resistors. The 100kW resistor could also be left off if you are sure that the RX pin will always be in a well-defined state. Construction options Construction We’ve specified a right-angled header for CON2 since that is what most CP2102-based modules are supplied with. If fitting the module to You’ll need all the standard gear for SMD work, including a good magnifier. This is one of the smaller projects we have created, and it packs the parts in fairly tightly. You might need a magnifier even to read the PCB’s silkscreen markings. Make sure you have solder flux (ideally as a paste), tweezers, a fine-tipped iron and a means of securing the board, such as Blu-Tack. Fume extraction (or working outside) will help remove flux smoke. You should also have a suitable solvent for cleaning up the PCB afterwards, and solder-wicking braid will be helpful in case a solder bridge forms. The USB-C Serial Adaptor is built on a double-sided PCB coded 24106241 that measures 16×22mm. We’ll refer Songbird An easy-to-build project that is perfect as a gift. SC6633 ($30 plus postage): Songbird Kit Choose from one of four colours for the PCB (purple, green, yellow or red). The kit includes nearly all parts, plus the piezo buzzer, 3D-printed piezo mount and switched battery box (base/stand not included). See the May 2023 issue for details: siliconchip.au/Article/15785 72 Silicon Chip Australia's electronics magazine siliconchip.com.au to the side with the USB-C socket as the top of the PCB, with microcontroller IC1 at the bottom. The overlay diagram, Fig.3, should help you place the small components. USB-C socket CON1 has the finest pin pitch of the parts used, so fit it first. Add a thin layer of flux to the PCB over its pads, then position the socket. There are holes to help align it, and you can add more flux to the top of the pins too. Tack the larger end-most pins and confirm that the other pins are aligned with their pads and that the socket is flat on the PCB. You can then solder the mounting pins to secure the location. Add flux to the tops of the mounting holes and apply the solder from below until it can be seen wicking up the pins to the top side. That way, you know this part is properly secured and won’t easily be torn off the PCB. Now solder the remaining pins of CON1. If you get a bridge between two pins, add a little more flux and use solder-wicking braid to draw it up. If you’re unsure about your soldering, clean up the flux to get a better view of the pins under magnification before proceeding. Solder REG1 in place next. This is on the same side of the PCB as CON1. Apply a little flux to the PCB pads and tack one lead, then check that the other leads are aligned before soldering them. That is the basic strategy needed for the remaining SMD parts. This side also has the three LEDs and their 1kW resistors. LED1 is blue and is fitted adjacent to the TX pin on CON2, while LED2 is red and is nearer to the RX pin. LED3 is green. While it wouldn’t be a tragedy if you mixed up the colours, we tried to make them easier to remember (eg, red and RX both start with the letter R). LED1 and LED2 have their cathodes towards the USB-C socket. The cathode is usually marked with a small green dot or something similar, but it’s best to check with a DMM set on diode test mode. When you touch the probes to the LED pads and it lights up, the red probe is on the anode, while the black probe is touching the cathode. LED3 faces the opposite direction. Next, solder the 1kW resistors and then one of the 1μF capacitors, which should be the last SMD part on this side of the PCB. Next, flip the PCB over and fit IC1. The technique is much the same, siliconchip.com.au Parts List – USB-C Serial Adaptor 1 double-sided PCB coded 24106241, 16×22mm 1 SMD USB Type-C socket with power & USB 2.0 data (CON1) [GCT USB4105 or equivalent] 1 6-way right-angle pin header (CON2) Semiconductors 1 PIC16F1455-I/SL microcontroller programmed with 2410624A.HEX, SOIC-14 (IC1) 1 blue SMD LED, M2012/0805 size (LED1) 1 red SMD LED, M2012/0805 size (LED2) 1 green SMD LED, M2012/0805 size (LED3) 1 MCP1700-3302 3.3V low-dropout linear regulator, SOT-23 (REG1) Capacitors (all M2012/0805 X7R, 10V or higher) 2 1μF 1 100nF Resistors (all SMD M2012/0805 size, 1/8W, 1%) 1 100kW 1 10kW 2 5.1kW 3 1kW 3 220W although its pins are smaller than those on the resistors and more closely spaced (although more widely spaced than the USB socket). Make sure you put it in the right way around, with pin 1 orientated as shown! Apply flux to the PCB, place the chip with tweezers and tack one lead. Check its alignment, then solder the other leads. It is best to fit the other 1μF capacitor next so that it doesn’t get mixed up with the 100nF capacitor that mounts next to it. The other seven parts are an assortment of resistors; ensure the correct values go in the right places, as shown in Fig.3. Now use a solvent to clean off any flux residue, allow the board to dry, then inspect it closely for bridges or dry solder joints. If everything looks good, you can solder your choice of CON2 and proceed with testing. Testing Try connecting the Adaptor to a USB supply. If you are not confident, don’t connect it to a computer, but use a USB power supply or something similar. You should see green LED3 illuminate within a second or so. If it does not, disconnect the Module and recheck the component placement and soldering. You could try flipping the USB-C cable to see if it makes any difference. If it does, that points to a problem with CON1 or the two 5.1kW resistors. While it is plugged into a power source, use a voltmeter to measure the 3.3V and 5V pins on CON2 relative to GND. A lack of 5V indicates a problem with CON1 or the 5.1kW resistors. Australia's electronics magazine If 5V is present but 3.3V is not, there could be a problem with the regulator, or perhaps another component is shorting the 3.3V rail. Once everything is working, connect the Adaptor to a computer and check that a new serial port is available. Use a program like TeraTerm or minicom to open the port and send some data by typing in the terminal window. You should see blue LED1 (next to TX) flash. If you connect the RX and TX pins on CON2 (eg, using a jumper cable) and send data, the red and blue LEDs should flash together as data is being looped back. Your terminal should echo the characters you are typing. If this is all as expected, the USB-C Serial Adaptor is working and can be deployed to your project. Using it The USB-C Serial Adaptor is generally a drop-in replacement for the CP2102 modules that it is intended to succeed. Like those modules, we use it to power and connect to projects for debugging purposes. We have also incorporated such modules into projects, such as the ESP32-CAM LCD BackPack (April 2024; siliconchip.au/Article/16212). You can use the USB-C Serial Adaptor instead of the micro-USB Type-B version specified in that project. Our Adaptor has some components on the bottom side, unlike the CP2102 modules, so it will need to be spaced away a little from the host PCB. The plastic insulation on standard pin headers should be sufficient for that purpose. SC June 2024  73